Atherosclerotic cardiovascular disease is a leading cause of death worldwide. Nevertheless, mechanisms of lesion development and progression are poorly understood. The overall goal of this proposal is to develop a detailed understanding of the dynamics of the annexin II system with respect to cardiovascular disease. Specifically, we hypothesize that the annexin binding partner, S100A10 (also known as p11), regulates fibrinolytic activity on the surface of the endothelial cell, the macrophage, and the vascular smooth muscle cell. In previous funding periods, we have demonstrated that annexin II, a Cadependent, phospholipid binding protein, is expressed on endothelial cells where it binds plasminogen and tissue plasminogen activator (t-PA), and accelerates the catalytic efficiency of plasmin generation. In recent data, annexin ll-null mice accumulate fibrin within blood vessels, fail to clear vascular thrombi, and display angiogenic defects. Although cell surface expression of annexin II is required for its profibrinolytic activity, it is not known how it translocates to the cell surface because it lacks a classical signal peptide. p11 is known to stabilize annexin II as a heterotetramer with enhanced membrane affinity, and new evidence suggests that p11 orchestrates the translocation of annexin II to the outer plasma membrane in response to heat stress or thrombin stimulation. p11 appears to facilitate tyrosine phosphorylation of annexin II by a Src-like kinase. New data also suggest that expression of p11, itself, is controlled by intracellular levels of annexin II;"orphan" p11 is degraded via an annexin ll-regulated ubiquitin-mediated proteasomal pathway. Our Specific Aims are to determine the mechanisms by which [1] p11 directs annexin II to the cell surface, [2] p11 regulates src-mediated phosphorylation of annexin II, [3] annexin II regulates the expression level of p11, and [4] p11 contributes to the vascular response to injury in vivo. These studies are expected to reveal important insights into the functional role of the annexin II fibrinolytic system in cardiovascular health and disease.